Visualizing polarization structure of lipid Langmuir monolayer by surface second-harmonic generation technique

•By using the second-harmonic generation microscope, we visualized Langmuir domains.•Anisotropic polarization structures inside the domains were observed.•Molecules at the cusps of the dendrite parts have larger tilt angle.•The molecules are oriented along the dendrite parts of domains.

By using optical second harmonic generation (SHG) technique, we visualized lipid Langmuir monolayers comprised of molecules with an SH active moiety, 1,2-dipalmitoyl-sn-glycero-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol- 4-yl) (NBD-PE), on focussing the shape and polarization structure of the domains. The domain shape visualized by the SHG was the same as that by the Brewster angle microscopy (BAM), while polar structure was identified at the cusps of dendritic domains. These results were found well supported by our theoretical prediction in our previous study.